A semiconductor quantum dot is a volume of semiconductor material different from that of the surrounding matrix with a dimension of typically less than 50 nm in all directions. The small size results in the confinement of carriers (electrons or holes, or both electrons and holes), and therefore the presence of quantum size effects, in all three dimensions. The observation of individual quantum dots, and the behaviour due to the quantum confinement in all three dimensions (such as changes in the temperature stability of the output power and the emission wavelength of devices) allows quantum dots to be distinguished from quantum wells, where quantum confinement only occurs in one dimension.
There is currently considerable interest in fabricating quantum dot light emitting devices in the aluminum gallium indium nitride (or (Al,Ga,In)N) material system. The (Al,Ga,In)N material system includes materials having the general formula AlxGayIn1-x-yN where 0<x<1 and 0<y<1. In this specification, “AlGaInN” denotes a member of the (Al,Ga,In)N system having non-zero amounts of Al, Ga and In, “InGaN” denotes a member having zero Al content and non-zero amounts of In and Ga, etc. The (Al,Ga,In)N material system can emit light in the ultra-violet, visible, and infrared parts of the electromagnetic spectrum. The use of quantum dots has several advantages over the use of quantum wells. The 3D confinement results in a weaker dependence of device characteristics with temperature as can be seen from FIG. 3a. FIG. 3a shows the output photoluminescence intensity of a light-emitting device having a conventional quantum well active region (squares) and a light-emitting device having a quantum dot active region (circles). In addition the very narrow density of states obtainable with a quantum dot active region owing to the 3D confinement results in a narrow gain spectra leading to significantly lower threshold currents for laser diodes. Obtaining a very narrow density of states requires a degree of uniformity of the quantum dots in the active region of the device.
FIG. 1 is a schematic sectional view of a typical semiconductor light emitting diode with a quantum dot active region fabricated in the AlGaInN material system. A n-type GaN buffer layer 2 is disposed on a substrate 1. An InGaN quantum dot layer 3a comprising InGaN quantum dots 3c is disposed on the buffer layer 2. Each quantum dot 3c has a limited extent in the x-, y- and z-directions. A GaN capping layer 3b is disposed on the InGaN quantum dot layer 3a. Further quantum dot layers 3a and capping layers 3b may then be grown to form a stack of InGaN quantum dot layers. An AlGaN electron-blocking layer 4 may be disposed on the final quantum dot layer 3a or the final capping layer 3b, or the AlGaN layer 4 may be omitted. A p-type GaN layer 5 may be disposed on the final quantum dot layer 3a, the final capping layer 3b or, if present, the AlGaN layer 4. With an InGaN quantum dot active region the device is able to emit light throughout the visible wavelength range and into the ultraviolet and infrared regions of the electromagnetic spectrum.
It is known to form self-assembled InGaN quantum dots as the active region of a light emitting diode. See for example Y K Su et al. Semicond. Sci. Technol. 19 (2004) 389-392. This document refers to an InGaN quantum dot LED.
A semiconductor quantum wire is a volume of semiconductor material different from that of the surrounding matrix with a dimension of typically less than 50 nm in two directions and a greater extent in the third direction. The small size in two directions results in the confinement of carriers (electrons or holes, or both electrons and holes), and therefore the presence of quantum size effects, in two dimensions.
Egawa et al disclose, in “High Performance InGaN LEDs on (111) silicon substrates grown by MOCVD”, IEEE Electron Device Letters, Vol 26, No. 3, pp 169-171 (2005), a light-emitting diode structure having a 20 nm thick n-Al0.27Ga0.73N layer disposed between the active region and the substrate. The AlGaN layer is separated from the active layer by a 20-pair AlN/GaN multilayer and a 0.2 μm thick GaN layer—that is, by a total of 700 nm.
JP-10 215 029, discloses providing an AlGaN layer 7 below an InGaN active layer such that there is a difference of at least +3% between the lattice constants of the two layers. This difference in lattice constant generates strain, and leads to three-dimensional island-shape growth such that an active layer containing islands or quantum dots can be achieved easily.